The present invention relates to remote sensing, tracking, and identification, and in particular to the production and use of inexpensive ID xe2x80x9ctags.xe2x80x9d
Various monitoring technologies are known and used to monitor the location of an article or to provide identification in a wide range of contexts. One such technology, known as xe2x80x9ctagging,xe2x80x9d is commonly employed, for example, in shoplifting security systems, security-badge access systems and automatic sorting of clothes by commercial laundry services. Conventional tagging systems may use some form of radio-frequency identification (RF-ID). In such systems, RF-ID tags and a tag reader (or base station) are separated by a small distance to facilitate near-field electromagnetic coupling therebetween. Far-field radio tag devices are also known and used for tagging objects at larger distances (far-field meaning that the sensing distance is long as compared to the wavelength and size of the antenna involved).
The near-field coupling between the RF-ID tag and the tag reader is used to supply power to the RF-ID tag (so that the RF-ID tag does not require a local power source) and to communicate information to the tag reader via changes in the value of the tag""s impedance; in particular, the impedance directly determines the reflected power signal received by the reader. The RF-ID tag incorporates an active switch, packaged as a small electronic chip, for encoding the information in the RF-ID tag and communicating this information via an impedance switching pattern. As a result, the RF-ID tag is not necessarily required to generate any transmitted signal.
Even though RF-ID tags have only a small and simple electronic chip, they are relatively complex devices requiring sophisticated manufacturing techniques to produce. A simpler alternative involves marker elements adapted to affect an interrogation signal in a measurable, characteristic way. Many such systems involve magnetic or magnetomechanical tags. For example, a magnetic wire or strip exhibiting harmonic behavior may be stimulated within an interrogation zone by transmitter antenna coils. The coils generate an alternating magnetic interrogation field, which drives the marker into and out of saturation, thereby disturbing the interrogation field and producing alternating magnetic fields at frequencies that represent harmonics of the interrogation frequency. The harmonics are detected by receiver antenna coils, which may be housed in the same structure as the transmitter coils. Accordingly, the appearance of a tagged article within the zonexe2x80x94which may be defined, for example, near the doors of a retail store or libraryxe2x80x94is readily detected.
While inexpensive, magnetic antitheft systems tend to encode very little, if any, information. Essentially, the tag merely makes its presence known. While some efforts toward enhancing the information-bearing capacity of magnetic tags have been madexe2x80x94see, e.g., U.S. Pat. Nos. 5,821,859; 4,484,184; and 5,729,201, which disclose tags capable of encoding multiple bits of dataxe2x80x94the tags themselves tend to be complex and therefore expensive to produce, and may require special detection arrangements that limit the interrogation range (the ""859 patent, for example, requires scanning a pickup over the tag) or involve specialized equipment.
The present invention utilizes tags having spatial inhomogeneities that encode information, and which may be detected in the time domain; in effect, characteristics in space are transformed into time for sensing purposes. Such tags may be very inexpensively produced yet carry appreciable quantities of data. Unlike the prior art, which requires specialized information-bearing structures, the present invention can utilize simple physical modifications to, or externally applied field biases operating on, materials that are very inexpensive to procure.
A first embodiment utilizes an elongated, amorphous, magnetically susceptible element, such as a magnetic wire. Along the length of the element, responsiveness to a time-varying magnetic field is altered in a spatial pattern corresponding to the information to be encoded. The element is then subjected to an interrogating magnetic field, and its response sensed over time to recover the spatially encoded information. It should be stressed that the harmonic tags described earlier can also take the form of magnetic wires that are subjected to interrogation signals. In such traditional systems, however, the signals are sensed in the frequency domain, not the time domain in order to provide a characteristic signature rather than information. The harmonics, in other words, merely facilitate unambiguous detection in an electromagnetically noisy environment.
Alternatively, instead of sensing the response of the element over time, amplitude and phase are detected and the time-domain information recovered from the phase. This is once again in contrast to traditional systems, which neither preserve nor analyze phase information.
In a second embodiment, the element exhibits magnetoelastic behavior, and once again the element is selectively modified either physically or by application of bias fields in accordance with a pattern of information. The element""s response to an interrogating magnetic field is sensed over time to recover the encoded information. For example, discrete bias fields applied to the element may define, along the length of the element, a plurality of segments responding differently to the applied field and producing intermodulating response signals. These response signals are sensed and analyzed in the time domain to characterize the bias fields and thereby read the information they encode.
Once again, magnetoelastic markers have previously been used for tagging purposes, but in a manner very different from that described herein. In particular, prior-art surveillance systems utilize only the fundamental mechanical resonance frequency of the marker. A representative marker includes one or more strips of a magnetoelastic material packaged with a magnetically harder ferromagnet (i.e., one with a higher coercivity) that provides a biasing field to establish peak magnetomechanical coupling. The mechanical resonance frequency of the marker is dictated essentially by the length of the strip(s) and the biasing field strength. When subjected to an interrogating signal tuned to this resonant frequency, the marker responds with a large signal field that is detected by a receiver. The size of the signal field is partially attributable to an enhanced magnetric permeability of the marker material at the resonance frequency.
In other prior-art systems, the marker is excited into oscillations by signal pulses, or bursts, generated at the marker""s resonance frequency by a transmitter. When an exciting pulse ends, the marker undergoes damped oscillations at its resonance frequency (i.e., the marker xe2x80x9crings downxe2x80x9d), and this response (ring down) signal is detected by a receiver. Accordingly, prior systems generally involve a single resonance frequency dictated by the entire tag structure, and a uniform bias field.
In a third embodiment, the element is a higher-frequency element (e.g., a UHF or microwave antenna) that is selectively modified either physically or by application of bias fields in accordance with a pattern of information. The modifications cause modulation to be introduced into the received signal, and the pattern of modulations is indicative of the modifications (and therefore the encoded information).
In a fourth embodiment, magnetic inhomogeneities are established with respect to an otherwise uniform NMR-responsive sample; for example, a magnetic bias strip may be disposed near or against the sample. The pattern of magnetic biases results in an NMR spectrum with multiple peaks corresponding to the bias pattern.